TC Vietnam: Groundwater Resources Assessment in Nam Dinh, Red River Delta

Report of the project:

Figure 1: Rural area in Nam Dinh, Red River Delta Source: BGR

Background:
Vietnam is in principle richly endowed with a subtropical to tropical climate and abundant water resources. However, impressive economic developments in recent years, combined with the population growth over the last two decades, have strongly increased pressure on water resources. Consequently, groundwater is increasingly important for Vietnam’s future water supply strategy, since surface water is vulnerable and increasingly affected by climate change, untreated sewage water and industrial waste water. Sustainable groundwater exploitation and management is crucial for life, development and the environment in Vietnam.

Figure 2: Areas of action of IGPVN project Source: BGR

Facing this background, the project “Improving Groundwater Protection in Vietnam” (IGPVN) was initiated as part of the Vietnamese-German technical co-operation. On behalf of the German Federal Ministry for Economic Cooperation and Development (BMZ), the Federal Institute for Geosciences and Natural Resources (BGR) advises Vietnamese authorities at a national and provincial level on aspects concerning exploration and monitoring of groundwater resources as well as its management and protection (Figure 2).

IGPVN project develops and implement strategies to tackle common consequences of the increasing and uncontrolled overexploitation of groundwater, such as:

Groundwater drawdown in a regional scale,
Salinization of coastal aquifers, and
Groundwater contamination as a result of e.g., percolating industrial and municipal waste water as well as agriculture and aqua farming.

The Nam Dinh province as a first pilot working area is facing declining groundwater heads as well as groundwater salinisation. The geoscientifc works and outcomes are summarized below. Future pilot areas and groundwater related issues will be Hanoi, and Ha Nam (groundwater exploration and salinization, Red River Delta), Quang Ngai (pollution, surface water – groundwater interaction, coastal aquifer) and Soc Trang (salinization and overexploitation, Mekong Delta).

Geological and Hydrogeological Frame:
Nam Dinh province is located in the south of the Red River flood plain on the South China Sea. The local geological structure is dominated by a complex sequence of unconsolidated high and low permeable alluvial and marine sediments. The geologic and geomorphologic development of the Red River Delta (RRD) was controlled by the glaci-eustatic sea-level change and the ongoing tectonic subsidence of the Red River Basin. Continuous accumulation of the sediment load has been interrupted by repetitive erosion events during glacial periods in the late Pleistocene and Holocene. Especially the last glacial event had a huge impact on both, distribution and interconnection of groundwater bearing strata in the subsurface of the RRD as well as the salinity of pore waters, as demonstrated below (see Conclusions).

Figure 3: Geological sketch map of South Red River Delta Source: BGR

The pre-neogene basement consists of Proterozoic crystalline as well as Triassic sedimentary rocks with increasing depth from West to East. Due to its close vicinity to the rim of the Red River basin, tectonic structures and features of the Western Nam Dinh are quite complex (Figure 3). One major structural feature in the subsurface of the Nam Dinh province is the uplifted Vu Ban block, indicated by few Proterozoic outcrops in the Northwest of the province. Another important structural feature is the incision of the Paleo-Song Hong (Red River) valley, which is, according to Tanabe et al. (2003), aligned East of Vu Ban Block and crossing Nam Dinh province to the sea (Figure 3).

Alluvial Pleistocene sediments of the Ha Noi Formation have proven to be yielding aquifers and are subject of extensive utilisation since few decades. Since the 1990s groundwater extraction exceeds recharge, resulting in groundwater drawdown up to 0.6 m/year and increasing salinization of the Pleistocene groundwater resources. Further relevant aquifers are located in sediments during Neogene and – in a lesser extend – Holocene time.

Application of Quantitative Hydrogeological Methods:
Since the installation of a groundwater monitoring network in Nam Dinh comprising 23 monitoring wells, groundwater quantity and quality in Holocene, Pleistocene, Neogene and Triassic aquifers is continuously monitored. The application of quantitative hydrogeological methods supplemented with archive data and previous studies, provided the database for the development of a conceptual hydrogeological model.

Figure 4: Cross section with depths profiles of gamma and induction well logging Source: BGR

Just to name some of the applied methods:

Hourly measurement of groundwater head, temperature, salinity and atmospheric pressure using absolute pressure transducer equipment.
Calculating hydraulic conductivity using comparative studies of lithologic and hydraulic methods. Slug test analysis in case of overdamped and underdamped aquifer response.
Time series analysis of hydraulic heads in confined aquifers to determine storage parameter based on barometric efficiency as well as observed tidal effects using simple analytical models.
Hydrochemical and isotopic studies addressing genesis, mixing, recharge and age of the pore water in the observed aquifers.
Geophysical induction well logging provides vertical formation conductivity profiles (Figure 4), which have been translated into salinity profiles based on own hydrochemical data and analytical diffusion modelling.
3D structural modelling (Figure 5) and numerical hydrogeological modelling including analysis of different extraction scenarios.

Figure 5: Boreholes (a) and network of cross sections (b) Source: BGR

Furthermore, knowledge-transfer in frame of training-on-the-job and workshops is a common practice in order to train local technical staff in the applied methods and data analysis.

Conclusions:
The Proterozoic horst in the NW of Nam Dinh is a major structural feature representing a hydraulic barrier between the Western Triassic hard rock aquifers and the eastern Red River Delta. Southward of this barrier, the unconsolidated Pleistocene and Neogene aquifers receive significant recharge from Triassic formations in West and NW of Nam Dinh. Furthermore, inflow from water-bearing fault zones in the basement is suggested by hydrochemical and stable isotope data. The influx during several ten thousand years has established a low saline water “lens” in Pleistocene and Neogene formations of very high drinking water quality (Figure 6).

Figure 6: Conceptual cross section with carbon-14 groundwater ages and salinity Source: BGR

A large portion of Holocene fine grained sediments have accumulated in a marine environment and, thus, contain saline pore water representing the major source for salinization due to diffusive transport into deeper aquifers. Diffusion modelling for 3000 year time span confirms high-saline Holocene pore waters as a major source and vertical diffusion as a major transport process, explaining the elevated salinity in brackish Pleistocene and even Neogene pore water in the Red River area East of Nam Dinh. In the West and Southwest, fresh and low saline pore water in qp and n aquifer only persist due to the continuous inflow of fresh water from the adjacent Triassic hard rock aquifers.

Figure 7: Map of arsenic in groundwater and a simple sand filter Source: BGR

However, in SE, East and NE of Nam Dinh, high saline pore water of marine origin still dominates the composition of groundwater in deeper aquifers. Moreover, the predominant reducing environment provides the frame for accelerating the mobilisation of redox-sensitive and potentially toxic substances, such as arsenic, ammonia, iron and manganese (Figure 7). Therefore, high yielding Pleistocene and Neogene aquifers in large areas of North, Eeast and SE Nam Dinh, are considered not to be usable for drinking water supply without applying appropriate water treatment technologies. Special caution to groundwater use must be spend on the transition areas of fresh to brackish salinity (1 g/L to 3 g/L) where groundwater consumption is still widespread due to lacking alternatives.

Increasing extraction and usage exceeding recharge of high quality Pleistocene groundwater in central and S Nam Dinh has huge impact on the natural geohydraulic system. A regional abstraction cone has been delineated in Pleistocene as well as the underlying Neogene aquifer. In this area, the natural coastward directed groundwater flow has turned towards the centre of the abstraction cone with horizontal apparent velocities of up to 0.6 m/a (up to 0.2 m/a in n unit). This suggests the migration of brackish and higher saline groundwater from East Nam Dinh as well as offshore towards the fresh groundwater area.

The lack of recent and reliable groundwater extraction data is a crucial handicap for understanding the water budget in qp aquifer and its replenishment. Vertical groundwater flow from and to Pleistocene might be low but cannot be omitted. Isotopic studies in Pleistocene and Neogene sediments suggest an apparent vertical flux of only 4 to 15 mm/a. Carbon-14 groundwater dating indicates a groundwater age of up to 12,000 years in Pleistocene aquifer in the centre of the abstraction cone (Figure 6). This confirms relatively slow recharge rates of the fresh groundwater area in Pleistocene and Neogene groundwater.

Figure 8: CWRPI staff taking groundwater samples Source: BGR

Results of the numerical modeling (see Technical Report, Part B) suggest that exploitation of the groundwater resources will not be reversible, as the demand for freshwater is way greater than the potential fresh groundwater recharge in the model area. With the current model settings, a freshwater intake of about 8000 m³/d stands in contradiction of an extraction of more than 60,000 m³/d. Sustainable groundwater resource management is probably not possible any more. Thus, a change of the strategy of water withdrawal must be considered. The current extraction habits will lead to a faster intake of saline water from the eastern and southern boundaries.

Based on the results discussed above, the IGPVN project formulates recommendations for water management and assists political decision makers in communicating and implementing these recommendations on a national and provincial level. In this context, an improved control of the numerous unregistered extraction wells represents a crucial task.

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